1. The Field of the Invention
The present invention relates generally to optoelectronic components. More particularly, the present invention relates to method for compensating for wavelength drift in a fiber-optic laser transmitter.
2. Background and Relevant Art
The proliferation of communication technologies creates every day increases in demand for data transfer channels. Optical networks are a highly-reliable and efficient way to satisfy this demand. As a result, there is a desire to achieve higher data throughput in existing optical networks. A current means for satisfying this desire is the use of Dense Wave Division Multiplexing (DWDM). As shown in FIG. 1, DWDM data from a plurality of sources is converted into optical signals 2 with different wavelengths by a plurality of optoelectronic transceivers 4. After being multiplexed by an optical multiplexer/demultiplexer 6, optical signals 2 may pass through a single optical cable 8 simultaneously, which greatly increases network throughput.
There are several standards for a signal traveling through an optical network. These standards specify acceptable wavelengths of a signal (channel) and the “distance” or spacing between neighboring channels. There exists a need, therefore, for optoelectronic transceivers capable of operating on specific wavelengths. Currently, the most popular standards are 200 GHz (gigahertz) spacing, which is equivalent to 1.6 nm (nanometers) spacing between neighboring channels, 100 GHz, equivalent to 0.8 nm spacing, 50 GHz, equivalent to 0.4 nm spacing, and 25 GHz equivalent to 0.2 nm spacing between channels. As described herein, the term “distance” in reference to channels refers primarily to the distance in frequency (Hz) or wavelength between channels in a spectrum, commonly one or more nanometers, rather than physical distance traveled, such as one or more miles, etc. The specific wavelengths (i.e., channels) acceptable for data transfer in an optical network are proscribed by the International Telecommunications Union (ITU).
Optical amplifiers, used to increase the strength of an optical signal before it enters an optical network, typically have an optimal operational wavelength range. For modern Ebrium-Doped Fiber Amplifiers (EDFA) the typical operational wavelength range is 1523 to 1565 nm. If the network is using a 200 GHz standard for channel spacing, the number of available channels is 22. For 100 GHz standard the number of channels is 45; for 50 GHz—90 channels; for 25 GHz standard—180 channels.
FIG. 2 shows a schematic representation of wavelength intervals when the channel spacing standard is 100 GHz. The distance between neighboring channel centers 10 is 0.8 nm. For a signal to stay within the allowed pass band 14 its wavelength must be within 0.1 nm of the center of the specified channel. Operation outside the allowable allowed pass band 14 results in high attenuation of the transmitted signal, and in some cases, potential cross-talk with an adjacent channel.
Unfortunately, the wavelengths of lasers tend to change over time for various reasons. The wavelength emitted by the laser may shift, for example, as the laser emitter ages or as the laser temperature increases. In order to calculate how much the laser emitter wavelength can shift before it starts encroaching on a neighboring channel, several parameters of laser emitter calibration are be taken into account.
When calculating the allowable pass band of a laser emitter, an allowance must be made for an initial setup tolerance 16 (FIG. 3) and temperature control tolerance 18. For example, for a part in which the initial wavelength is targeted at the center channel, and with a set-up tolerance of +/−10 pm (picometers), and a temperature control tolerance of +/−20 pm, for a combined set-up and temperature control tolerance of +/−30 pm. Based on these tolerances and a 100 pm maximum total wavelength offset tolerance, the allowable wavelength aging is +/−70 pm over the life of the part.
There are several factors determining the wavelength of a signal produced by traditional laser sources. These factors include current density, temperature of the laser emitter, as well as specific inherent characteristics of the laser emitter. The relationship between the temperature of the laser emitter and the wavelength produced is typically around 0.1 nm/° C. for Distributed Feedback (“DFB”) sources that are commonly used in DWDM applications. This means that if the laser emitter temperature is increased by 10° C., the wavelength of the emitted light will shift about +1 nm.
Since the wavelength produced by a transceiver at a specified laser emitter temperature and current density differs from one laser emitter to the other, the optoelectronic transceivers are initially calibrated before being installed in an optical network. The calibration includes monitoring the wavelength of optical signals produced by the laser emitter while varying its temperature as well as other operating conditions, and then storing calibration information in the memory of a microprocessor. It also includes receiving analog signals from sensors in the optoelectronic device and converting the analog signals into digital values, which are also stored in the memory. As a result the device generates control signals based on the digital values in the microprocessor to control the temperature of the laser emitter. The method of calibrating an optoelectronic transceiver is described in detail in a U.S. patent application entitled “Control Circuit for Optoelectronic Module with Integrated Temperature Control,” identified by Ser. No. 10/101,248, and filed on Mar. 18, 2002, which is incorporated herein by reference.
For performance and reliability reasons, it is desirable to operate a laser emitter at a temperature between 15° C. and 50° C. There are several factors limiting the acceptable range of operating temperatures. First, a laser emitter ages more rapidly when operated at temperatures above 50° C., and may cause reliability concerns at typical end of life conditions (20–25 years). The quantum efficiency of the laser emitter decreases with age and, therefore, forces the transceiver to operate at higher currents in order to provide a fixed optical power, which further accelerates the aging of the laser emitter. In addition, temperature performance characteristics of the device used to control the laser temperature determine the lower limit of the available range of temperatures. A well-designed thermal system using a single stage thermo-electric cooler (TEC) as a temperature control device can typically provide up to 40° C. cooling. Since the standard maximum operating temperature of a transceiver is 70° C., the 40° C. cooling capability of the TEC means that the effective operating range of the laser emitter in the transceiver is restricted to temperatures between 30° C. and 50° C.
Finally, persons skilled in the art recognize that the wavelength of a laser diode varies during its operational lifetime. As a result, steps need to be taken in order to ensure that the wavelength does not drift outside of a selected channel during this operational lifetime. Prior art techniques for preventing this drift include the use of wavelength lockers, which are expensive and of questionable reliability.